Dielectric resonators are used in many circuits, particularly microwave circuits, for concentrating electric fields. They can be used to form filters, oscillators, triplexers, and other circuits.
FIG. 1 is a perspective view of a typical dielectric resonator of the prior art. As can be seen, the resonator 10 is formed as a cylinder 12 of dielectric material with a circular, longitudinal through hole 14. While dielectric resonators have many uses, their primary use is in connection with microwaves and, particularly, in microwave communication systems and networks.
As is well known in the art, dielectric resonators and resonator filters have multiple modes of electrical fields and magnetic fields concentrated at different center frequencies. A mode is a field configuration corresponding to a resonant frequency of the system as determined by Maxwell's equations. In a dielectric resonator, the fundamental resonant mode frequency, i.e., the lowest frequency, is the transverse electric field mode, TE01δ (hereinafter the TE mode). Typically, it is the fundamental TE mode that is the desired mode of the circuit or system into which the resonator is incorporated. The second mode is the hybrid mode, H11 (or H11, hereafter). The H11 mode is excited from the dielectric resonator, but a considerable amount of electric field lays outside the resonator and, therefore, is strongly affected by the cavity.
FIG. 2 is a perspective view of a dielectric resonator filter 20 of the prior art employing a plurality of dielectric resonators 10. The top wall (cover) is removed in the Figure to reveal the components of the filter. However, typically, of course, the housing 24 is completely enclosed. The resonators 10 are arranged in the cavity 22 of a conductive housing 24. Conductive tuning plates 42 may be positioned above the resonators 10 to permit adjustment of the center frequency of the resonators. The conductive housing 24 commonly is rectangular, comprising six planar external walls.
Microwave energy is introduced into the cavity via an input coupler 28. The energy may then be coupled to a first resonator (such as resonator 10a) using a coupling loop. Conductive separating walls 32 separate the resonators from each other and block (partially or wholly) coupling between the resonators 10. Specifically, conductive material within the electric field of a resonator essentially absorbs the field coincident with the material and turns it into current in the conductor so that the field does not pass through to the other side of the wall. In other words, conductive materials within the electric fields cause losses in the circuit. Hence, conductive walls without irises generally prevent all coupling between the resonators separated by the walls, while walls with irises 30 permit a controlled amount of coupling between adjacent resonators.
Conductive adjusting screws 33 coupled to the floor 26 of the housing 24 may be placed in the irises 30 to further affect the coupling of the fields between adjacent resonators and provide adjustability of the coupling between the resonators. When positioned within an iris, a conductive adjusting screw partially blocks the coupling between adjacent resonators permitted by the iris. Inserting more of the conductive screw into the iris reduces coupling between the resonators while withdrawing the conductive screw from the iris increases coupling between the resonators.
Tuning plates 42 may be provided adjacent each resonator mounted on adjusting screws 44 passing through the top cover (removed and not shown in FIG. 2 to permit viewing of the components of the circuit 20) of the enclosure 24.
In a typical dielectric resonator circuit, such as a filter, the resonators are allowed to couple to each other in one particular order. For instance, in the microwave filter illustrated in FIG. 2, the energy from the input coupler 28 couples to the first resonator 10a. Resonator 10a couples to resonator 10b through the iris 30a in wall 32b, resonator 10b also couples to resonator 10c through the iris 30b in wall 32c, resonator 10c couples to resonator 10d through the iris 30c in internal wall 32d, etc. Longitudinal separating wall 32a contains no iris and therefore prevents cross coupling between any other pairs of resonators. The internal walls 32b, 32c, 32d also prevent other cross coupling, such as resonators 10a and 10c and resonators 10b and 10d. 
A coupling loop connected to an output coupler 38 is positioned adjacent the last resonator 10d to couple the microwave energy out of the filter 20.
In some dielectric resonator filter circuits, it may be desirable to provide for cross coupling between otherwise non-adjacent resonators. This may be desirable in order to adjust the bandwidth (or rejection) of the filter. Specifically, the sizes of the resonators 10, their relative spacing, the number of resonators, the size of the cavity 22, the size of the irises 30, and the size and position of the tuning plates 42 and/or tuning screws 33 all have some effect on (and need to be controlled to set) the desired center frequency of the filter, the bandwidth of the filter, and the rejection in the stop band of the filter. The bandwidth of the filter is controlled primarily by the amount of coupling of the magnetic fields between the various dielectric resonators, which is largely a function of the distances between the coupling resonators and the size of the irises (or other opening) between the resonators. Generally, the more coupling between the individual resonators, the wider the bandwidth of the filter. On the other hand, the center frequency of the filter is controlled in large part by the size of the resonator and the size and the spacing of the tuning plates 42 from the corresponding resonators 10.
In order to permit cross coupling of the electromagnetic fields between resonators that would not otherwise exist due to distance and/or the separating walls 32, a cross-coupler 34 comprising a conductive element, such as a coaxial cable, can be provided that extends through a hole or slot 25 in one or more of the separating walls 32 between two dielectric resonators, e.g., resonators 10a and 10c. If desired in order to obtain more optimum filter transfer functions, the cross coupler can be prevented from making conductive contact with the housing by a non-conductive bushing 34a. The non-conductive bushing 34a would electrically isolate the probe 34b from the housing 24 so that electric fields coincident to the probe 34b are not absorbed by the walls of the housing, but rather are passed from one end of the cross coupler 34 to the other for coupling resonators adjacent the ends of the cross coupler 34.
A detailed discussion of cross-coupled dielectric resonator circuits is found in U.S. Pat. No. 5,748,058 to Scott entitled CROSS COUPLED BANDPASS FILTER.
As previously noted, it may be desirable to alter the amount of cross coupling provided through the cross coupling element 34 in order to tune the bandwidth or rejection of the filter. In the past, this has been done manually by opening the housing and physically bending the cross coupling elements to move it closer to or farther from the corresponding resonator(s). This is a laborious and time-consuming process because it typically requires the removal of one of the walls to permit access to the cavity. The housings typically are constructed of one removable wall attached by a large number of screws, not uncommonly several dozen. Thus, simply opening the housing to gain access to the cavity might require unscrewing 20, 30, 40, or even more screws, which then, after tuning, of course, need to be tightened again in order to enclose the housing. Since tuning is an imprecise process, commonly, the filters will then be tested to see if the desired bandwidth or rejection has been achieved. If not, the screws would need to be removed again, the wall removed, the cross coupling element re-adjusted, the wall replaced, the screws reattached, and the filter tested again.
In addition, typical necessary adjustments in the position of the end of the cross coupling element might be on the order of hundredths or even thousandths of an inch. Accordingly, performing such adjustments by bending the cross coupling element by hand or even with tools, can be extremely difficult.